Method for manufacturing a fuse and a fuse made by that method

ABSTRACT

A method for manufacturing a fuse comprising a fuse element (10), in the form of a single wire, which is formed together with at least one terminal (11,11a,11b), in the form of a number of twisted wires. The fuse element and the conductor(s) are formed from the same wire, whereby part of the wire is moved back and forth a number of times to form at least one ring (21) or loop. The ring or loop is twisted into a conductor (11,11a,11b) in the form of a number of twisted strands, whereby one of the twisted strands is formed to be included, in part, in the terminal (11,11a,11b) twisted together with an additional number of wires and to constitute, in part, a fuse element (10).

TECHNICAL FIELD

The present invention relates to a method for manufacturing a fusecomprising a fuse element in the form of a wire and at least oneterminal, the terminal having a larger cross section area than the fuseelement.

BACKGROUND ART

A fuse comprises a fuse element, for example in the form of a wire,which at elevated current is heated and melts, the fuse blows, and thecomponent that the fuse protects is disconnected.

In certain applications, such as fuses for protection of capacitorelements in power capacitors, the fuse comprises, in addition to fuseelements, also terminals which have a larger cross section area than thefuse element.

The fuse element usually consists of a wire, preferably a metal wire ofcopper, aluminium, silver, alloys based on these metals, as well asnickel silver, or any other material with a resistivity and meltingpoint suitable for the application.

For a fuse element provided with terminals, the contacting, that is, thetransition between terminal and fuse element, which with conventionaltechnique is usually performed by means of soldering, welding or amechanical pressure joint, is a critical step. The influence of heatfrom the soldering/welding or mechanical damage upon the contacting bymeans of the pressure joint weakens the fuse, preferably at thetransition between terminal and fuse element. This has an adverse effecton the fuse and the protective function by the introduction ofweakenings or other inhomogeneities in the fuse.

In connection with fuses in capacitor elements or in other componentswhere the fuse is mounted "narrowly" and in close proximity toelectrically loaded materials, all forms of mechanical pressure pointsmust be avoided. For that reason, the transition between terminal andfuse element in fuses which make contact with or are mounted in closecontact with electrically loaded materials, for example fuses mountedinside or adjacent to a capacitor element, must be of essentially thesame dimension as the terminal, that is, no lumps of solder, weldingmaterial, mechanical clamps or the like, which exceed the dimensions ofthe terminal and create pressure points, can be allowed.

The invention aims to propose a method whereby a fuse, which comprises afuse element in the form of a thin wire and at least one terminal, theterminal having a larger cross section area than the fuse element, canbe manufactured in an economic and rational way while at the same timeavoiding the above-mentioned problems arising in connection with thecontacting or the transition between terminal and fuse element.

SUMMARY OF THE INVENTION

The invention relates to a method for manufacturing a fuse whichcomprises a fuse element in the form of a thin wire and at least oneterminal, the terminal having a larger cross section area than the fuseelement. The fuse element and the terminal are formed in one piece fromthe same wire while moving the wire, section-by-section, back and fortha number of times to form at least one ring or loop. The ring obtainedis then twisted into a terminal in the form of a number oftwisted-together wires, whereby one of the wires, included in part inthe terminal, is formed also to partly constitute a fuse element. Themethod according to the invention simplifies the manufacture of fuses inrelation to the known technique since no joining together of differentcomponents is required and all material, wire, is taken from the sameroll.

According to a preferred embodiment of the method of the invention,fuses comprising two terminals and an intermediate fuse element aremanufactured from the same wire. According to this embodiment, the wireis moved, section-by-section, back and forth a number of times to format least one ring or loop. The ring thus formed is twisted to form alarger cross section area than the wire. Alternately with these sectionsof twisted wires, sections in the form of a single wire are arranged,whereupon the body formed from this wire, comprising sections in theform of a single wire, arranged alternately in the longitudinaldirection with sections in the form of a number of twisted-togetherwires, are cut into fuses. By performing the cutting in the twistedsections, fuses comprising two terminals and an intermediate fuseelement are obtained.

Since soldering, welding or mechanical joints are not needed in themanufacture of fuses with a method according to the invention, theproblems arising in connection with the critical transition betweenterminal and fuse element are avoided.

In the manufacture of fuse elements using a method according to theinvention, the fuse element and the terminal are preferably manufacturedstarting from a thin metal wire with a diameter smaller than 0.5 mm.Especially advantageous are fuses where the fuse element and theterminal are made in one piece from a thin wire of copper, aluminium,silver, alloys based on these metals, as well as electroplated nickelsilver or any other metal or alloy having a resistivity and meltingpoint suitable for the application.

To provide an inproved fuse of the type specified herein, it isadvantageous to construct fuses from a thin metal wire. With aconventionally designed fuse where the contacting between the fuseelement and the terminal is performed by means of welding, soldering ormechanical pressure contact, metal wires with a diameter smaller than0.3 mm cannot normally be used. In the manufacture of a fuse accordingto the method of the invention, metal wires with a diameter smaller than0.3 mm are used to advantage.

A fuse manufactured according to the method of the invention is used toadvantage in a power capacitor comprising a number of capacitorelements, each capacitor element being connected in series with aninternal fuse which comprises a fuse element, and at least one terminal,wherein a thin wire which constitutes the fuse element according to theinvention is adapted, in part, to be included in the terminal twistedtogether with an additional number of wires.

The invention is particularly valuable in the manufacture of fusesintended to be used in a power capacitor which comprises a number ofsub-capacitors or capacitor elements which are protected by means ofso-called internal fuses since in these power capacitors each capacitorelement is connected in series with its own fuse. For the fuses tooccupy a minimum space, they are mounted inside the capacitor,preferably inside or in close proximity to the individual capacitorelements. The fuses are thus mounted clamped between electrically activematerial, whereby any form of mechanical pressure point, such as solderlumps, welding material, or mechanical joints, is undesirable. A fusemanufactured by means of a method according to the invention isadvantageously performed starting from a thin wire and is free fromsoldering, welding or mechanical joints at the contact point betweenfuse and terminal.

The dimensions of an internal fuse in a power capacitor are determinedby two opposing criteria, namely that

--the current load from the current which is estimated to flow throughthe fuse when the sub-capacitor to which it is series-connected isshort-circuited shall be so large that the wire melts, and that

--at the same time the fuse is to withstand the current load whicharises when the entire capacitor and hence all the capacitor elementsare discharged. To limit the energy load on the fuse, it is an advantageif the fuse terminals can be dimensioned such that part of the dischargeenergy is absorbed in the fuse terminals. A fuse designed according tothe present invention can be optimized in a simple and rational way tomeet the above-mentioned demands for resistance and energy absorptioncapacity in the conductors in relation to those in the fuse element. Theenergy absorption capacity and resistance of a terminal depend, inaddition to the dimension and material of the wire, on the length of theconductor, the number of twisted wires, strands, in the terminal, aswell as the pitch during the twisting operation. On the basis of therequirements described above, it has proved to be particularly suitablewith terminals in the form of three or five strands, corresponding toone or two rings or loops, in fuses which are to be used for internalfuses in power capacitors. To optimize the resistance and energyabsorption capacity of the terminals in relation to the resistance andenergy absorption capacity of the fuse element, with fuses manufacturedaccording to the current technique, it is required that the terminals,which are designed as separate parts in the form of wires, plates, etc.,are manufactured in a number of different dimensions to be able to becombined with fuse elements of different dimensions and rated currents.

The use of a fuse, manufactured according to the method of theinvention, as an internal fuse in a power capacitor results in thefollowing advantages:

--the negative influence in the form of inhomogeneities in the materialproperties, which arise by the action of heat during welding/solderingor by mechanical damage upon contacting with clamping joints, at thetransition between terminal and fuse element, is avoided;

--pointwise mechanical load, from electrically loaded material, frompressure points, i.e. solder lumps, welding material, mechanical clampsor the like, at the transition between terminal and fuse element, isavoided;

--the resistance and energy absorption capacity of the terminal can beoptimized, in a simple and rational way, in relation to thecorresponding properties of the fuse element;

--an improvement of the protective function is obtained by the use of athinner wire in the fuse element; and

--capacitor designs with thinner fuses can be selected.

In addition to the above enumerated advantages, a rational basis for themanufacture of fuses is also obtained, as generally is the case forother types of fuses.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail in the following withreference to the accompanying drawings. FIGS. 1 and 2 show fusesmanufactured by means of the invented method. FIGS. 3a and 3b show theinvented method for the manufacture of fuses, and FIGS. 4a and 4b showthe use of fuses, manufactured according to the invented method, inpower capacitors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a fuse comprising a fuse element 10 in the form of a thinwire and at least one terminal 11. The problems arising in connectionwith the contacting 12 or the transition between the terminal 11 and thefuse element 10 are avoided in that the wire, which constitutes the fuseelement 10, is also adapted to form part of the terminal 11 twistedtogether with an additional number of wires formed in a ring or loop (asdescribed below). The conductor 11 has been produced by moving the wire,which also constitutes the fuse element 10, in those parts whichconstitute the terminal, back and forth a number of times to form atleast one ring or loop, and by twisting this ring or loop together withpart of the wire which also constitutes the fuse element 10 into aterminal 11.

FIG. 2 shows a fuse comprising two terminals 11a, 11b with anintermediate fuse element 10 in the form of a wire. The terminals 11a,11b have a larger cross section area that the fuse element 10. The thinwire which constitutes the fuse element 10 is also adapted to form partof the terminals, twisted together with an additional number of wiresformed in a ring or loop (as described below). The terminals 11a, 11bhave been produced by moving the wire, which constitutes theintermediate fuse element 10, in those parts which constitute theterminal back and forth a number of times to form at least one ring orloop, and by twisting this ring or loop together with at least part ofthe wire which also constitutes the fuse element 10 into a terminal.

Since soldering, welding, or mechanical joints have not been used forthe contacting 12 or the transition in fuses according to FIGS. 1 and 2,the problems arising in connection with the critical transition betweenterminals 11, 11a, 11b and fuse element 10 are avoided.

The fuse element 10 and the terminals 11, 11a, 11b are preferably madefrom a thin metal wire with a diameter smaller than 0.5 mm. Particularlyadvantageous are fuses in which the fuse element 10 and the terminals11, 11a, 11b are made in one piece from a thin wire of copper,aluminium, silver, an alloy based on any of these metals, as well aselectroplated nickel silver or any other metal or alloy with aresistivity and melting point suitable for the application.

From the point of view of protective function, it is advantageous withfuses made of thin metal wire. With a conventionally designed fuse wherethe contacting 12 between the fuse element 10 and the terminals 11, 11a,11b is performed by means of welding, soldering, or a mechanicalpressure contact, metal wires with a diameter smaller than 0.3 mm cannotnormally be used, whereas a fuse designed according to the presentinvention is advantageously designed from metal wires of a diametersmaller than 0.3 mm.

FIG. 3a shows the invented method for the manufacture of a fuseaccording to FIG. 1, the fuse element 10 and the conductor 11 being madefrom the same wire by moving the wire back and forth to form at leastone ring 21 or loop. The ring or rings 21 obtained are twisted into aterminal 11, whereby also part of the wire, which is adapted toconstitute the fuse element 10, is twisted together with the wiresincluded in the ring. The manufacture of fuses according to the inventedmethod simplifies the manufacture of fuses in relation to the knowntechnique since no joining together of different components is requiredand all material, wire, is taken from the same roll.

A preferred embodiment of the method according to the invention, inwhich fuses comprising two terminals and an intermediate fuse element,in the form of a single wire, are manufactured from the same wire isshown in FIG. 3b. The wire, which is preferably fed from a material roll25, is moved section-by-section back and forth a number of times to format least one ring 21, or loop. The rings 21 formed are twisted togetherinto sections 26 which have a larger cross section area than the wire.Alternately with these sections of twisted-together wires, sections 27in the form of a single wire are arranged. After the wire has beenformed into a body, comprising sections 27, in the form of single wire,arranged alternately in the longitudinal direction with sections 26, inthe form of a number of twisted wires, the body is cut in the twistedsections 26 as shown at 28. Cut parts are arranged to form fusesaccording to FIG. 2, comprising two terminals 11a, 11b and anintermediate fuse element 10.

FIGS. 4a and 4b show the use of a fuse, manufactured according to themethod described above, in a power capacitor.

The use of a fuse, manufactured by a method according to the invention,in a power capacitor comprising a number of capacitor elements 30 inwhich each capacitor element is connected in series with an internalfuse 31, which comprises a fuse element and terminals, and in which athin wire which constitutes the fuse element is also adapted to beincluded, in part, in the terminals twisted together with an additionalnumber of wires, is particularly valuable.

Power capacitors comprising a number of sub-capacitors 30 or capacitorelements are shown in FIGS. 4a and 4b. The capacitor elements 30 areso-called wound foil capacitors, composed of a plurality of wound turnsof metal foils, serving as electrodes, and a solid dielectric arrangedbetween the turns, preferably in the form of polymer film or paper. Thewound foil capacitors are arranged stacked on top of each other and areelectrically connected together. The stacks of capacitor elements 30 arearranged enclosed in a container 35 which is provided with electricbushings 36 for connection of the power capacitor. For discharge of thepower capacitor, discharge resistors 37 are arranged.

The capacitor elements 30 are protected by means of fuses 31. In powercapacitors with internal fuses 31, each capacitor element 30 isconnected in series with its own internal fuse 31, which means that thefuses 31 have to be mounted inside or in close proximity to thecapacitor element 30 in such a way as to occupy a minimum space. Thefuses 3 are preferably mounted between two capacitor elements 30.Alternatively, as shown in FIG. 4b, a number of fuses are mounted closeto each other between plates of an electrically insulating material, forexample pressboard plates.

The fuses 31 are thus mounted clamped between electrically loadedmaterial, whereby any form of mechanical pressure point, such as solderlumps, welding material, or mechanical joints, is negative. With amethod according to the invention, a fuse 31 may advantageously be madestarting from a thin wire and without soldering, welding, or amechanical joint at the contact point between the fuse and theterminals.

The dimension of an internal fuse 31 in a power capacitor is determinedby two opposite dimensioning criteria, namely that

--the current load from the current which is estimated to flow throughthe fuse 31 when the sub-capacitor 30 to which it is series-connected isshort-circuited shall be so large that the wire melts, and that

--at the same time the fuse 31 is to withstand the current load whicharises when the entire capacitor and hence all the capacitor elements 30are discharged. To limit the energy load on the fuse 31, it is anadvantage if terminals can be dimensioned such that part of thedischarge energy is developed in the terminals. A fuse 31 designedaccording to the present invention can be optimized in a simple andrational way to meet the above-mentioned demands for resistance andenergy absorption capacity in the conductors in relation to those in thefuse element. The energy absorption capacity and resistance of aterminal depend, in addition to the dimension and material of the wire,on the length of the terminal, the number of twisted wires, strands, inthe terminal, as well as the pitch during the twisting operation. On thebasis of the requirements described above, it has proved to beparticularly suitable with terminals in the form of three or fivestrands, corresponding to one or two rings or loops, in fuses which areto be used for inner fuses 31 in power capacitors. To optimize theresistance and energy absorption capacity of the terminals in relationto the resistance and energy absorption capacity of the fuse element,with fuses manufactured according to the current technique, it isrequired that the terminals, which are designed as separate parts in theform of wires, plates, etc., are manufactured in a number of differentdimensions to be able to be combined with fuse elements of differentdimensions and rated currents.

The use of a fuse, manufactured according to the method of theinvention, as an internal fuse 31 in a power capacitor results in thefollowing advantages:

--the negative influence in the form of inhomogeneities in the materialproperties, which arise by the action of heat during welding/solderingor by mechanical damage upon contacting with clamping joints, at thetransition between terminal and fuse element, is avoided;

--pointwise mechanical load, from electrically loaded material, frompressure points, i.e. solder lumps, welding material, mechanical clampsor the like, at the transition between terminal and fuse element, isavoided;

--the resistance and energy absorption capacity of the terminal can beoptimized, in a simple and rational way, in relation to thecorresponding properties of the fuse element;

--an improvement of the protective function is obtained by the use of athinner wire in the fuse element; and

--capacitor designs with thinner fuses can be selected.

The use of fuses manufactured by a method according to the invention inpower capacitors has been described above only referring to powercapacitors built up from capacitor elements in the form of so-calledwound foil capacitors. Fuses according to the invention are, of course,suitable also in other types of power capacitors.

We claim:
 1. A method for manufacturing a continuous string of fusesfrom a single strand of fuse wire, each fuse having at least one fuseelement and at least one terminal, comprising the steps of:forming saidat least one fuse element from said single strand of wire; moving saidsingle strand of wire back and forth over a portion of said singlestrand of wire forming said fuse element to form at least one ring orloop; twisting said at least one ring or loop to form said terminal withsaid ring or loop at the junction of said fuse element and saidterminal; and successively repeating said step of moving and twisting toform the continuous string of fuses.
 2. A method according to claim 1,wherein said single strand of wire has a diameter smaller than 0.5 mm.3. A method according to claim 1, wherein said single strand of wire ismade from one of the following group of elements consisting of copper,copper alloy, silver, silver alloy, aluminum, aluminum alloy andelectroplated nickel silver and having a diameter less than 0.3 mm.
 4. Amethod according to claim 1, further comprising the step of cutting thecontinuous string of fuses substantially at the midpoint of eachterminal to form individual fuses.
 5. A method according to claim 1,wherein each fuse includes first and second terminals interconnected bysaid fuse element and said first terminal is formed by said steps ofmoving and twisting, and further comprising the additional step ofmoving said single strand of wire back and forth over another portion ofsaid single strand of wire forming said fuse element to form at leastone other ring or loop; and twisting said fuse element, said firstterminal and said other ring or loop to form said second terminal withsaid other ring or loop at the junction of said fuse element and saidsecond terminal; and successively repeating said step of moving andtwisting, said additional step of moving and said steps of twisting toform the continuous string of fuses.
 6. A method according to claim 5,wherein said single strand of wire has a diameter smaller than 0.5 mm.7. A method according to claim 5, wherein said single strand of wire ismade from one of the following group of elements consisting of copper,copper alloy, silver, silver alloy, aluminum, aluminum alloy andelectroplated nickel silver and having a diameter less than 0.3 mm.
 8. Amethod according to claim 5, further comprising the step of cutting thecontinuous string of fuses at substantially the midpoint of said secondterminal to form individual fuses comprising said first and secondterminals and said fuse element.
 9. A continuous string of fuses havingat least one fuse element formed of a single strand of said singlestrand of wire, comprising:at least one fuse element formed from saidsingle strand of fuse wire; at least one terminal partially formed bymoving said single strand of wire back and forth over a portion of saidsingle strand of wire forming said fuse element to form at least onering or loop; said terminal being completely formed with said ring orloop at the junction of said fuse element and said terminal by twistingsaid at least one ring or loop; and the continuous string of fuses beingformed by successively repeating said step of moving and twisting.
 10. Acontinuous string of fuses according to claim 9, wherein said singlestrand of wire has a diameter smaller than 0.5 mm.
 11. A continuousstring of fuses according to claim 9, wherein said single strand of wireis made from one of the following group of elements consisting ofcopper, copper alloy, silver, silver alloy, aluminum, aluminum alloy andelectroplated nickel silver and having a diameter less than 0.3 mm. 12.A continuous string of fuses according to claim 9, wherein the at leastone fuse is obtained by cutting the continuous string of fusessubstantially at the mid-point of each terminal.
 13. A continuous stringof fuses according to claim 9, wherein each fuse includes first andsecond terminals interconnected by said fuse element and said firstterminal is formed by said steps of moving and twisting and said secondterminal is formed by moving said single strand of wire back and forthover another portion of said single strand of wire forming said at leastone fuse element to form at least one other ring or loop; said otherring or loop is formed at the junction of said fuse element and saidsecond terminal by twisting said fuse element, said first terminal andsaid other ring or loop; said continuous string of fuses being formed bysuccessively repeating said step of moving and twisting, said additionalstep of moving and said steps of twisting.
 14. A continuous string offuses according to claim 13, obtaining individual fuses comprising saidfirst and second terminals and said fuse element by cutting thecontinuous string of fuses at substantially the mid-point of said secondterminal.